When a mechanical structure is the seat of periodic vibrations, or noises, a known general procedure consists in engendering and applying to this mechanical structure, by means of an actuator, a compensation vibration or noise, in phase opposition with the original noise or vibrations. This method is called the active antivibration method.
Effecting the control which will produce the damping of the vibrations or noises, that is to say the generation of the compensation vibration or noise, is tricky since the degree of damping or cancellation of the original vibration ultimately obtained depends on the accuracy in amplitude and in phase of the compensation vibration.
The currently proposed devices, for the reasons indicated previously, do not give full satisfaction.
Among the latter, two operating modes have been proposed.
A first operating mode, illustrated by FIG. 1a, consists in synthesizing the compensation vibration harmonic by harmonic, by using a synchronizing signal arising from the periodic vibration itself.
Such an operating mode has been described in particular by the international patent application WO 83/01525 published on Apr. 28, 1983.
In such an operating mode, the calibration, in amplitude and in phase, of the harmonics is performed in such a way as to cancel out the acceleration signal .gamma. measured by means of an accelerometer placed on the structure to be protected.
The disadvantage of the abovementioned operating mode is that the latter is not suited to non-stationary noise and vibration phenomena since the convergence time of the system is relatively long; furthermore, and for this reason, the synthesis of the compensation vibration can be performed only on a restricted discrete number of harmonics, this hampering the accuracy in amplitude and in phase of the reconstituted compensation vibration, in relation to the original vibration, and ultimately the degree of attenuation of the latter.
The second operating mode proposed hitherto consists in making no assumption about the nature or waveform of the original vibration, the latter, as represented in FIG. 1b, being detected and measured by means of a second sensor delivering a signal representing the incident acceleration .gamma.i. This signal is processed by an auto-adaptive filter with variable coefficients, F.A.A., delivering the compensation vibration, the detection and measurement of the residual acceleration .gamma.r then making it possible to match the coefficients of the filter so as to render the signal for controlling the compensation vibration a minimum. The abovementioned operating mode therefore employs a mode of temporal processing.
A disadvantage of this operating mode is that all the signals engendered by the sensors are eventually processed by the system, in the absence of synchronization in regard to the periodic character of the original vibration. Such a disadvantage is liable to lead to major problems when, for example, the noise level is significant, or when the natural resonant modes of the mechanical structure, when the latter is suspended, are excited, the corresponding device not being in any case in a position to perform a corresponding discrimination with a view to the processing of the original vibration alone.